Field emission devices that generate electron beams from electron emitters such as carbon nanotubes at an anode plate are well known in the art. Each of the electron beams are received at a spot on the anode plate and define a corresponding spot size. The separation distance between the cathode plate and the anode plate determine, in part, the spot size. It is known in the art to control the spot size by using focusing structures to collimate the electron beams.
To achieve adequate display brightness, the anode should be maintained at a high voltage relative to the cathode. Consequently, the anode plate and cathode plate should be spaced far enough apart that nanotubes due not emit electrons from the anode field alone, and the spacers that separate the anode and cathode plates do not break down. This separation distance that is sufficient to prevent unwanted electrical events can result in an undesirably large spot size. As the voltage on the cathode plate generates the electrons which are attracted by a gate electrode from the emitter toward the anode plate, the gate electrode voltage tends to cause the beam to diverge. Thus, focusing structures are frequently employed in field emission devices.
However, prior art focusing structures often employ dielectric layers to support a focusing electrode and to separate the focusing electrode from the other electrodes, such as gate extraction electrodes, of the field emission device. Such prior art focusing structures suffer from disadvantages. For example, the capacitance between the focusing structures and the gate extraction electrodes increases the power requirements of the device. Many focusing structures add additional layers and processing steps to reduce the beam spot size, and this significantly increases fabrication cost and reduces yield. Processing geometries are reduced for many methods, thus reducing the available emitter area, increasing the sensitivity to defects, and increasing processing and equipment costs associated with decrease in feature sizes. For example, a typical focusing scheme incorporates a separate perforated sheet placed between the anode and cathode. The extra sheet adds costs, the precision alignment step reduces yield, and the device requires a double set of spacers, which is extremely complex and costly due to difficulty and low yield.
Prior art focusing structures also reduce the size of the region that can incorporate electron emitters, thereby reducing the overall device current. Prior art focusing structures have also been placed too close to the emitting material, thereby screening the emitter material at the edge of the emitter material region entirely from the gate field. While this reduces the beam size, it also reduces the overall current by orders of magnitude. Prior art focusing structures which place focusing electrodes in proximity to the electron emitters with a distance less than the average height of the emitters risk having the emitters stick to the focusing structure via van der Waals forces, thereby rendering the emitters less useful. Many prior art focusing concepts do not describe a method for implementation and typically require significantly more patterning layers than are economically viable. Prior art focusing structures often do not address the importance of the relationship of the focusing structure to the height of the emitting features. They are typically proposed for electron focusing and fail to account for the re-orientation of the actual emitting nanostructures in the electric field.
Accordingly, it is desirable to provide a field emission device structure that focuses the electron beam from the emitter area away from the gate extraction field without significantly increasing the device cost, thereby providing for a small spot size required for high resolution displays. Such a device would require no new patterning steps, no increased complexity, and would not require more complicated spacer technology. It is desirable for the structure to orient the emitter structures more perpendicular to the anode, thereby reducing the initial lateral velocity. It would also be advantageous to create a structure which increases the focusing function for regions with the highest emission current, since these are regions that would see the largest absolute intensity away from the beam center. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.